In a simple Rankine cycle system, such as an automotive Rankine cycle engine system, there is a liquid pump for pumping a working fluid to a required boiler pressure, one or several heat exchangers for heating and evaporating the working fluid, one or more expansion machines for converting heat energy of the working fluid into mechanical energy, and a condenser for liquefying the working fluid vapors exhausted from the expansion machines.
One embodiment of an organic Rankine cycle system includes a working fluid and a lubricant which are separated and separately used in the system. The system is designed such that the working fluid and the lubricant are separated during a certain operation mode of the system. However, in another operation mode the working fluid and the lubricant do come into contact with each other. And, when the working fluid and the lubricant do come into contact with each other, the working fluid can dilute the lubricant; and the viscosity of the lubricant can be significantly reduced (e.g., to less than (<) 20 percent (%) of its initial value) rendering the use of the lubricant ineffective. When contact of the working fluid and the lubricant and dilution occurs, it is critical that the working fluid and the lubricant be miscible with each other because working fluids are known to have poor lubrication characteristics and this can cause severe wear and, in extreme cases, equipment damage.
In a Rankine cycle the mechanical parts of an engine transform the pressure of a working fluid into mechanical energy such as an expansion machine. The mechanical parts of the engine are required to be lubricated. One class of lubricant known in the art is a polyalkylene glycol (PAG) lubricant. Also known in the art is the use of an alcohol as a working fluid in a Rankine system. For example, a specific alcohol that is used as a working fluid can be ethanol. Currently, Rankine cycle systems are being designed such that the working fluid used in the system can be a mixture of water and alcohol such as ethanol; and wherein the water concentration in the working fluid is typically from 1 percent by weight or weight percent (wt %) to 50 wt %.
Combinations of PAGs as a lubricant and ethanol as a working fluid have been reported in WO2014/128266. The engines described in WO2014/128266 allow working fluid to enter the lubricant space and the PAGs are diluted with ethanol which reduces the viscosity of the PAGs, however if the viscosity of the lubricant is too low (e.g., <20% of its original value), friction and wear of the engine parts can increase to an unacceptable level resulting in wear debris and severe equipment damage. Wear and equipment damage are also possible when the viscosity of the lubricant is reduced to greater than (>) 50% of its original value. It is preferred that the viscosity is not reduced by >30% of its original value and most preferred if the viscosity change is <10% of its original value.
Machines with moving parts that deal with pressurized fluids (working fluids) such as pumps, compressors, and engines need lubrication of the moving parts. Heretofore, several different options have been used for providing such lubrication such as: (1) the working fluid used has minimal lubrication properties but is expected to perform some minimal lubrication; (2) the working fluid used is designed to contain additives (mainly lubricity additives) that are dissolved in the working fluid; (3) the lubricant and the working fluid are blended to form an emulsion as a single phase; and (4) the lubricant and the working fluid are physically separated by using a sealed system. Using option (4) in practice, usually results in the lubricant and the working fluid inadvertently coming into contact with each other.
As aforementioned, when the working fluid and lubricant are in contact, the viscosity of the lubricant can be significantly reduced rendering the use of the lubricant ineffective. For example, when the working fluid is an alcohol and water mixture and such working fluid contacts the lubricant, when the lubricant is for example a polyalkylene glycol, the viscosity of the lubricant is reduced to a greater extent (e.g., to up to about 75%).
The choice of working fluid used is normally based on the physical and thermal process requirements of the Rankine cycle system used. Typical working fluids include for example hydrofluorocarbons (HFCs), alcohols and alkanes which have good thermal properties but have poor lubrication properties. One disadvantage with options (2) to (3) above is that the working fluid cannot be used in systems where high temperatures (e.g., greater than [>]150° C.) are experienced because the lubricant or lubricant additives can degrade quickly at these temperatures such that acceptable maintenance intervals cannot be achieved. Therefore, for systems with temperatures over 150° C. option (4) above is a preferred option of choice. However, with use of option (4), the lubricant needs to provide wear protection of the moving parts of the system across the operating temperature range. But one disadvantage with option (4) is that the physical sealed system separating the lubricant and the working fluid can fail or working fluid and lubricant are blended when the system shuts down; and then the working fluid can enter the area of system containing the lubricant. This can affect the viscosity and the wear protection of the lubricant. For example, if the viscosity of the lubricant is reduced too much (e.g., to <80% of its original value), then the lubricant film cannot fully separate moving mechanical parts and wear of surfaces occurs in the system.
In automotive Rankine cycles alcohols and HFCs are often the working fluids of choice because the thermal properties of the alcohols and HFCs maximize efficiency and minimize equipment cost. See for example the description in Dieter Seher et. al., “Waste Heat Recovery for Commercial Vehicles with a Rankine Process”, 21st Aachen Colloquium Automobile and Engine Technology 2012. Most common lubricants such as mineral oils, polyalpha-olefins, alkylated naphthalenes, and some esters are not miscible with the alcohol and HFC working fluids (especially HFCs). In this case, the working fluid can displace the lubricant on critical parts to be lubricated in the equipment (e.g., shafts, cylinders, bearings etc.) and this can increase friction and wear of the critical parts that can lead to equipment failure.
However, even if a working fluid that dissolves in the lubricant is selected, such working fluid may negatively affect other properties of the lubricant, for example, the viscosity of the lubricant can be lowered (e.g., to <80% of its original value) to the detriment of the equipment.